1. Field of the Invention
This invention relates to a laser light emitting device used for fine working or as a light source for a light exposure device or as an illuminating device.
2. Description of Related Art
Heretofore, fifth harmonics of a laser light beam were generated by the laser of a low repetition frequency and high peak power. In case of solid-state lasers, the usual practice is to use a substantially collimated light beam, with the beam diameter being set to a larger value for evading damage to the crystal for wavelength conversion, using pulses of an extremely high laser power and a repetition frequency of the order of 1 to 100 pulses per second. However, this system has scarcely been put to practical use because of serious damages to crystals (see R. Kato, Researches in Laser. Vol.18, pp.3 to 7, W. Wiechmann et al., Post Deadline Papers of CLEO, 1995, CPD 19, and K. Deki et al., Extended Abstracts to Lectures of the Society of Applied Physics, 57th Autumnal Meeting, 7-a-M-9, pages 827). Therefore, proposals for optimizing the efficiency are lacking. In case of a UV light source for a light exposure device, a high repetition frequency is meritorious in view of removal of speckles. Except for proposals for maximizing the efficiency in each stage of waveform conversion or controlling the beam shape (S. Kubota et al., Patent Application entitled "Optimization of Beam Focus of UV Laser", there again lacks a proposal for optimising the overall efficiency.
That is, importance has not been attached to the problem of quantitative optimization due to the fact that the fifth harmonics are generated after three stages of waveform conversion so that the optical system for producing the fifth harmonics tends to be complex, the optical components tend to be damaged on generation of the UV light, and that fifth harmonics have not been used extensively for industrial applications.
The generation efficiency of the fifth harmonics is not that high because of many stages of waveform conversion. Therefore, if desired to apply the fifth harmonics to industrial use, it is necessary to maximize the generation efficiency of the fifth harmonics. If the generation efficiency of the fifth harmonics is to be maximized, that is if 100%-conversion is done in the extreme case, the basic wave cannot be additively frequency-mixed with the fourth harmonics generated from the second harmonics thus reducing the efficiency to 0%. Therefore, there must exist an optimum efficiency of generation of second harmonics. However, this quantitation has not been achieved to data.